Applicant claims the priority of Japanese patent application, Ser. No. 11-024203, filed Feb. 1, 1999.
1. Field of the Invention
This invention relates to a carburetor for a two stroke engine and more particularly to a carburetor which provides a scavenging air supply independently of a fuel and air mixture.
2. Background of the Invention
In a conventional rotary valve type carburetor, a fuel and air mixture is delivered to the engine through a single bore or mixing passage having a throttle valve. At least in part to reduce engine hydrocarbon emissions, it has been proposed to provide a scavenging air supply to the engine through a second bore in the carburetor. However, at least at engine idle operation, the scavenging air supply is undesirable as it provides too much air to the engine causing an undesirably lean fuel and air mixture in the engine and poor engine performance, stability and acceleration. Further, using a flat circular flap or butterfly type air valve to control air flow through the second bore does not provide a sufficient seal of the second bore even when the valve is in its fully closed position such that some air still flows through the closed valve. Still further, a carburetor of a type wherein the air valve is connected to the throttle valve by means of a lever and a link will be undesirably large in size and expensive to manufacture and assemble.
For a two stroke engine, a carburetor with an air passage, a separate fuel and air mixing passage and an air valve and a throttle valve connected together by a lost motion coupling. The coupling connects a valve shaft of the air valve with a valve shaft of the throttle valve and has a zone wherein the valves are rotatable relative to each other corresponding to a difference between an opening and closing angle of the air valve and an opening and closing angle of the throttle valve. Preferably, when the throttle valve is in an idle position, the air valve is closed and the throttle valve can be opened through at least some angle from its idle position before the air valve is moved from its closed position. Also, the air valve is preferably fully opened when the throttle valve is moved to its fully or wide open position. Thus, at idle operation of the engine, the air valve is closed to prevent a scavenging air supply from being provided to the engine to avoid an overly lean fuel and air mixture in the engine. When the engine accelerates sufficiently off idle, the air valve is opened to supply the scavenging air supply to the engine which facilitates removal of exhaust gases and reduces the escape of a fresh fuel and air mixture through the engine exhaust ports.
Additionally, according to another object of the invention, a circumferential edge of the air valve is inclined or angled with respect to a generally flat face of the valve in order to increase the effectiveness of a seal created by the engagement between the air valve and carburetor body when the air valve is in its closed position.
Objects, features and advantages of the invention include providing a carburetor which supplies a fuel and air mixture to an engine and selectively provides a scavenging air supply to the engine, terminates the scavenging air supply at low speed and low load engine operation, permits timing of the initial opening of the air valve relative to the throttle valve of the carburetor to be freely set, has an air valve with an improved seal with the carburetor when in its closed position, selectively interconnects a throttle valve and an air valve, permits at least some relative movement between the throttle valve and air valve, does not interconnect the throttle valve and air valve with a lever and link arrangement, avoids providing too much air to the engine, improves engine operation, reduces engine emissions, increases engine fuel efficiency, is of relatively simple design and economical manufacture and assembly, reliable and in service has a long useful life.